Abstract
Strain-controlled low-cycle fatigue testing on axial coupon samples of coiled tubing alloys have revealed that the modulus of elasticity for cyclically stabilized material is considerably lower (e.g., on the order of 20%) than the modulus measured during the initial load application or during a conventional monotonic tensile test. However, a closer analysis indicates that the majority of the modulus drop occurs during the first strain application. Additional axial testing with small elastic unload-reload excursions validate that increasing plastic deformation during an otherwise monotonic load application causes an increasing drop in the elastic modulus. In this paper, fatigue data from two coiled tubing materials are presented which demonstrate the profound influence of plastic deformation on the modulus of elasticity. The modulus decrease is explained on the basis of fundamental reordering, or texturing, within the crystal structure of individual steel grains. The lower modulus has important implications with regards to depth estimation, buckling calculation and stuck point calculation.
